Recent efforts have been made to improve bearings for high-speed rotating machinery, such as gas turbines for aircraft engines, missile engines or power generation (auxiliary power units). Such efforts have led to the development of various designs for fluid film hydrodynamic bearings. Generally, fluid film hydrodynamic bearings have been successfully employed in high-speed rotating machines for about the past twenty years. For example, air cycle machines used for aircraft cabin environment control systems utilize fluid film hydrodynamic bearings. Such bearings generally operate on the principle that a high-speed rotating member, such as a shaft, journal, or thrust runner, and an adjacent element, such as a compliant foil or the like, establish and maintain a pressurized fluid film therebetween. Moreover, such bearings operate on the principle that the high-speed rotating member is at least slightly eccentric with respect to rotation about its longitudinal axis. Therefore, if the rotating member is enclosed by a close-fitting, compliant, annular element such as a thin foil, or multiple such foils, encased within a stationary retaining member (sometimes referred to as a sleeve, a cartridge, a retainer, a bushing or a base), the eccentricity of rotation within such retaining member will form and maintain the pressurized fluid film layer, sometimes referred to as a fluid film wedge, between the rotating member and the compliant foil. The fluid film layer, in turn, provides a lubricated support for the rotating member.
More specifically, the high-speed rotation of the rotating member generates a high pressure in the fluid film layer, which fluid film supports the load imposed by the rotating shaft. A resilient backing member—e.g., a spring foil—is often disposed between a smooth, compliant foil and the stationary retaining member to accommodate deflections of the compliant foil resulting from pressurization, centrifugal forces and temperature differentials caused within the retaining member in order to maintain optimum or at least adequate fluid film layer geometry. Desirable fluid film hydrodynamic bearings have high load capacity and high coulomb damping for suppression of shaft whirl and excursions of the rotating member due to bearing loading and imbalances. Providing such desired characteristics for optimal performance has typically required stringent control of manufacturing tolerances of the fluid film hydrodynamic bearing.
One type of known fluid film hydrodynamic bearing is a multi-pad type as described in U.S. Pat. No. 3,615,121 to Barnett et al. and U.S. Pat. Nos. 4,153,315, 4,178,046 and 4,195,395 to Silver et al. Such prior art multi-pad bearings commonly have an iris-type construction. Moreover, the foils are typically unidirectional and overlapping. Generally, coulomb damping, which is required to suppress whirl of the rotating member, is low for such multi-pad bearings, and the low damping levels have limited the utilization of such bearings for high-speed rotating machinery. The three aforementioned patents to Silver et al. further teach stiffener elements for the smooth foils.
U.S. Pat. No. 4,178,046 discloses a foil bearing assembly in which a plurality of smooth foils is mounted within the retaining member or bushing, each subtending a rotational segment, less than all, of the circumferential or inner surface of the retaining member. Each foil comprises two sides or wings extending in opposite directions from a midpoint. Each foil is mounted at its midpoint with one side or wing of the foil serving as an underfoil for the overfoil of an adjacent foil and the other arm or wing serving as an overfoil for the underfoil of an adjacent foil on its opposite side. The arrangement of the foils is such that the sliding travel or shifting of adjacent foils (which results from the forces imposed on the foils by the pressurized fluid film generated by the rotating shaft) is in the same direction. Consequently, the relative sliding travel between adjacent foils is the difference between the amount of sliding travel of each foil. This limited relative foil movement contributes to the low coulomb damping characteristic of these multi-pad bearings. In order to compensate for such limited coulomb damping levels, the art often provides multi-pad bearing foils having a preformed diameter—i.e., the foil diameter prior to insertion of the rotating shaft into the bearing—which is up to 50% less than the diameter of the shaft. Consequently, when the shaft is initially mounted within the bearing, the bearing foils maintain a relatively tight grip on the shaft. This results in a high preloading on the shaft and thereby requires a high starting torque for the rotating shaft. If any type of contaminant, such as water, is present in the bearing, a still higher starting torque is required. Such high starting torque is, of course, disadvantageous as it stresses the machinery being used to drive the shaft and may be severe enough to result in inability to start the engine or motor driving the shaft and/or cause wear or damage to the engine, motor or other drive components. The tight grip of the bearing foil in the shaft also makes the foil susceptible to rotational deflection and slipping during start up and operation of the rotating shaft.
Improved bearings have been provided in the form of reverse 360-degree multi-layer hydrodynamic fluid film foil bearings described in U.S. Pat. Nos. 4,415,280 and 4,415,281 issued to G. L. Agrawal, incorporated herein by reference. In these bearings, two layers of smooth compliant foils are arranged to shift in opposite rotational directions. The smooth foils are supported on a layer of corrugated foil which serves as a resilient spring foil and provides high load capacity for the bearing. Due to the fact that adjacent foils shift in opposite directions, coulomb damping is relatively high because the relative movement between adjacent foils is equal to the sum of the individual foil movements. Accordingly, adequate coulomb damping is attained without the necessity of reducing the preformed diameter of the foils to significantly less than that of the shaft. Consequently, preloading imposed on the shaft by the foils is small and the starting torque required is not significantly increased by the bearing. However, because the foils are a single-pad type, and thus are supported at only one end thereof, extending for 360-degrees around the entire circumference of the inner surface of the stationary retaining member, the foils of these otherwise successful bearings occasionally telescope during assembly and operation. If the foils should telescope during operation, the telescoped foils tighten around the shaft and bind it. Ultimately, this leads to bearing failure. Further, manufacture of such 360-degree single-pad foils is expensive as it requires extensive manual operations to position the foils so as to hold the required tolerances during operation.
Improved bearings were provided in the form of reverse multi-pad hydrodynamic fluid film foil bearings described in U.S. Pat. No. 5,634,723 issued to G. L. Agrawal, incorporated herein by reference. The key features of a reverse multi-pad bearing are as follows. A retaining member has an inner surface, which defines a shaft opening within which a rotatable shaft is receivable for rotation. A foil assembly lines the inner surface of the retaining member and comprises a plurality of foil sub-assemblies, the foil sub-assemblies each subtending a rotational segment, less than all, of the inner surface. The foil sub-assemblies comprise compliant contact foils disposed in overlying contact with spring foils which are disposed radially outwardly of their associated contact foils. The spring foils and the contact foils are affixed to the retaining member in respective opposite-facing rotational directions along the inner surface. With this arrangement, sliding travel of the spring foils is in the opposite rotational direction of sliding travel of the contact foils. This design provides the high coulomb damping similar to the reverse 360-degree multi-layer foil bearing but requires a higher preload to keep the bearing assembly together during bearing installation and operation. This preload is still less than the original multi-pad bearing designs, as described by U.S. Pat. No. 3,615,121 to Barnett et al. and U.S. Pat. Nos. 4,153,315, 4,178,046 and 4,195,395 to Silver et al. Additionally, manufacturing of reverse multi-pad hydrodynamic fluid film bearings is simpler than the previous designs of hydrodynamic fluid film bearings with challenges only coming from assembly of the components into the retaining member or bushing.
In all of the prior art bearing designs, be they multi-pad or single-pad types, same direction or reverse direction, there is susceptibility of deflection of the foils at the point where they are supposed to be held in the retaining member. Specifically, during rotation of the shaft, there is a tendency for the foils to “pop up” or radially deflect, which compromises compliancy of the bearing. Such deflection occurs despite an end of each of the foils often being mounted, in part, within the stationary retaining member. In many designs, an end of the foil is disposed within a channel or keyway formed in the inner surface of the retaining member to position the foils as well as to prevent axial slipping of the foils when the rotating member is rotating. Commonly, the foil is attached to a key, pin or other support slidably disposed within the channel as shown in U.S. Pat. No. 3,615,121 to Barnett and U.S. Pat. Nos. 4,415,280 and 4,415,281 to Agrawal. Alternatively, the end of the foils can be bent to fit into a channel. However, heretofore, the bent or shaped end of such foil has typically been designed to permit some radial movement of the foil, as, for example, exhibited in the pivotally disposed foils shown and described in U.S. Pat. No. 4,348,066 to G. L. Agrawal, incorporated herein by reference. Moreover, such “hinged” foils place a greater emphasis on the manufacturing process of the bearing—notably, if the size of the “hinge” is too small, the foil will be loose in the channel and lead to a loss of positive bearing retention.
Accordingly, there has long been a desire to design such bearings and foil assemblies to prevent radial deflection while keeping the bearing loaded to desired level so as to maintain desired compliancy of the bearing foils. At the same time, it is further desirable that such bearings and foil assemblies restrict rotational slipping and telescoping within the retaining member so as to maintain desired compliancy, load capacity and coulomb damping levels.